Spark plugs and other such ignition devices used in internal combustion engines are subjected to high temperature environments produced in the combustion chambers. The high temperature environment often takes a toll on the different components of the spark plug and, over time, can cause diminished performance of the spark plug. One way in which the spark plug performance is negatively affected involves the conductance of the center wire assembly, whose components are used to deliver an electrical ignition pulse from the plug's terminal input to its spark gap. Corrosion, breakdown of materials and other phenomena accelerated by the extreme heat can negatively affect the conductive characteristics of these components, thus altering the intensity of the ignition pulse and ultimately the spark delivered to the combustion chamber. This effect can be of particular concern in spark plugs that utilize resistive noise suppression to reduce EMI (electromagnetic interference) from the plug.
The use of resistive noise suppression devices is well known in spark plugs. One type of these noise suppressors is commonly referred to as a capsule resistor. An older example of the use of capsule resistors can be seen in U.S. Pat. No. 2,906,909 issued to Somers et al. The spark plug shown in this patent includes a center wire assembly having (starting from the spark gap and moving axially upwards towards the ignition lead receptacle) a center electrode, a conductive glass seal, a metal contact, a capsule resistor element, a contact spring and a threaded contact cap. The conductive glass seal is a fired in conductive seal (FICS) that provides a gas tight seal while permitting the discharge energy to be conducted through the glass seal.
Another known type of spark plug noise suppressor is a resistive glass seal that is often used in addition to or in lieu of a conductive glass seal to provide both the gas tight seal within the insulator center bore as well as a resistive path for the spark discharge energy to reduce electrical interference. This resistive glass seal is a fired in suppressive seal (FISS) and can provide the benefits of both the conductive glass seal and capsule resistor in a single component. But as with most components, with advantages come certain drawbacks. In certain high temperature environments, such as natural gas engines or Formula One engines where temperatures in the center wire assembly can exceed 700° F., a noise suppressing glass seal can exhibit an electrical resistance that increases over time as a result of the elevated operating temperatures, and can even reach a point at which the seal behaves as an open circuit.
Thus, it would be advantageous to provide an improved center wire assembly for use in a spark plug that permits the use of a noise suppressing glass seal in a manner that provides some protection of the glass seal from the high heat environment of the combustion chamber.